In a projection optical system of a conventional projection exposure apparatus, a plurality of optical elements constituting the projection optical system are supported by a support member by means of an optical element holding member. A conventional optical element holding member supports the peripheral portion of an optical element at three points arranged at equal angular intervals. In order to ensure the accuracy of the projection exposure apparatus, each optical element must be held stably to enable accurate adjustment of its position in the direction of the optical axis and its attitude such as the tilt angle with respect to the optical axis. Therefore, a technique has been developed to dynamically support the optical element with a movable support member, which is driven by an actuator. The support member is driven in accordance with environmental changes to finely adjust the attitude and position of the optical element in the direction of the optical axis.
There is a recent demand for a high-resolution projection exposure apparatus in order to produce highly integrated semiconductor devices. As shown in the next equation, a projection exposure apparatus has a resolution Res that depends on the wavelength λ of the exposure light and a numerical aperture NA of a projection optical system:Resolution Res=k·λ/NA, where k is a constant.
In other words, in order to enhance the resolution of a projection exposure apparatus, the numerical aperture NA must be increased, and the wavelength of the light source must be shortened. The shortening of the light source wavelength has made progress as in a KrF excimer laser (λ=248 nm), an ArF excimer laser (λ=193 nm), and an F2 excimer laser (λ=157 nm). In order to shorten the light source wavelength, optical elements are formed of a material having high transmittance such as quartz glass or calcium fluoride. Another known technique increases the transmittance by filling the interior space of a barrel with nitrogen or helium.
In order to satisfy the demand for performing fine processing, such as 45 nm process for DRAMs and 32 nm process for MPUs, EUVL (EUV Lithography) using EUV light (Extreme UltraViolet light in soft X-ray region) having a wavelength of about 5 to 20 nm is necessary. Since an ordinary optical lens or a lens formed of a material such as quartz glass or calcium fluoride cannot be used to perform EUVL, a projection optical system must be formed by using a reflection mirror arranged in a vacuum. The use of the reflection mirror makes it possible to use light having an extremely short wavelength like EUV light without reducing the transmittance. Examples of a projection optical system applicable to an exposure apparatus used for EUVL technology are described in patent document 1 and patent document 2.
Patent Document 1: U.S. Pat. No. 6,485,153
Patent Document 2: U.S. Patent Application Publication No. 2004/0125353
As for a refraction system in which optical elements, which form a projection optical system like in the prior art, are used as transmission lenses, the refraction system has a rotation-symmetric shape with respect to an optical axis. Thus, a plurality of the optical axes for the transmission lenses may be arranged along the center line of a barrel. Accordingly, if the optical elements have a satisfactory weight balance, they can be stably supported by, for example, arranging three support members at equal angular intervals along their circular peripheral portions.
However, when a reflection mirror is used as the optical element constituting a projection optical system to shorten the light source wavelength, a light path must be bent within the projection optical system. Therefore, the reflection mirror is inclined with respect to the optical axis of the optical element. In addition, the reflection mirror may be partially cutaway to form the light path. Such a cutaway portion makes the reflection mirror rotationally-asymmetrical to the optical axis.
An asymmetrical optical element will be poorly balanced even if it is supported at three points along the peripheral portion at equal angular intervals by a conventional optical element holding member. Therefore, an optical apparatus having such asymmetrical optical element lacks static and dynamic stability and is susceptible to the influence of environmental vibrations or may exhibit an unpredictable behavior during alignment adjustment of the optical elements. Thus, the conventional optical element holder has been one of factor that lowers the accuracy of a projection exposure apparatus used in an operation for manufacturing highly integrated semiconductor devices.